1. Field of the Invention
The invention relates to a reflection-type liquid crystal display having a reflecting plate for reflecting out a light transmitted through a liquid crystal layer from an outside and a method for manufacturing a same.
The present application claims priority of Japanese Patent Application No. 2000-013216 filed on Jan. 21, 2000, which is hereby incorporated by reference.
2. Description of the Related Art
Reflection-type liquid crystal displays have been used mainly in a portable terminal because they can be made thinner, less power consuming, and lighter in weight than transmission-type ones. Specifically, a reflecting plate in the reflection-type liquid crystal display reflects an incident light transmitted from the outside, and it is therefore available as a light source for display, thus eliminating a necessity of a back-light.
A recent reflection-type liquid crystal display includes basically a liquid crystal of, for example, a TN (Twisted Nematic)-type, a single polarizing plate-type, a STN (Super Twisted Nematic)-type, a GH (guest host)-type, or a PDLC (Polymer Dispersion)-type, a Cholesteric-type, or alike, a switching element for driving the liquid crystal, and a reflecting plate provided inside or outside a liquid crystal cell. Such a typical reflection-type liquid crystal display employs an active matrix scheme which realizes high definition and high picture quality by using a TFT (TFT) or metal/insulator film/metal-structured diode (MIM) as the switching element and also has the reflecting plate attached thereto.
The following will describe a conventional liquid crystal display of the single polarizing plate-type with reference to FIG. 19.
An opposed-side substrate 1 includes a polarizing plate 2, a phase-difference plate 3, a glass substrate 4, a color filter 5, and a transparent electrode 6. A lower side substrate 7 includes, on the other hand, a glass substrate 8, a reverse stagger-structured TFT 9 formed as a switching element on the glass substrate 8, a protrusion shape 10 made up of a first insulating layer which provides an unevenly-structured base, a polyimide film 11 formed thereon as a second insulating layer, and a reflecting electrode 13 which is connected to a source electrode 12 of the TFT 9 and also which functions as a reflecting plate-and-pixel electrode.
Between the opposed-side substrate 1 and the lower side substrate 7 is located a liquid crystal layer 14.
A reflected light 16 is utilized for display. The reflected light 16 is given by an incident light 15 from outside when it passes through the polarizing plate 2, the phase-difference plate 3, the glass substrate 4, the color filter 5, the transparent electrode 6, and the liquid crystal layer 14 and then is reflected by the reflecting electrode 13.
This reflection-type liquid crystal display needs to have such display performance that it would give bright and white display when the liquid crystal is in a light-transmitting state. To achieve this display performance, the incident light 15 transmitted in various orientations needs to be efficiently emitted to the outside. To do so, an uneven structure can be formed on the polyimide film 11 to thereby provide the reflecting electrode 13 located thereon with a light-scattering function. Therefore, the display performance of the reflection-type liquid crystal display largely depends on how to control the uneven structure of the reflecting electrode 13.
The following will describe a conventional method for manufacturing a reflecting electrode used in the conventional reflection-type liquid crystal display with reference to FIG. 20A and FIG. 21J.
In steps for manufacturing a TFT, first a gate electrode 21 is formed on a glass substrate 20 (FIG. 20A). Subsequently, a gate insulator film 22, a semiconductor layer 23, and a doping layer 24 are formed (FIG. 20B). Subsequently, an island 25 of the semiconductor layer 23 and the doping layer 24 is formed (FIG. 20C), thereby forming a source electrode 26 and a drain electrode 27 (FIG. 20D). Next, a reflecting electrode 34 is formed.
In steps for manufacturing the reflecting electrode, first an organic insulator film 28 is formed which has photosensitivity (FIG. 20E). Subsequently, protrusions 29 are formed by photolithography in a region for forming the reflecting electrode (FIG. 20F) and melted into a smooth protrusion shape 30 (FIG. 21 G). Subsequently, the smooth protrusion shape 30 is covered with an organic insulator film 31 to form a further smoother uneven surface 32 (FIG. 21H). Subsequently, to electrically connect the reflecting electrode to the source electrode of the TFT, a contact portion 33 is formed (FIG. 21I), to then form a reflecting electrode 34 (FIG. 21J). This method for manufacturing reflecting electrodes is disclosed for example in Japanese Examined Patent Application No. Sho 61-6390 and in Proceedings of the SID (Tohru Koizumi and Tatsuo Uchida, Vol. 29, p. 157, 1988).
FIG. 22 is a plan view of a pattern of the protrusions 29 in the FIG. 20F. The following will describe the process with reference to FIG. 22F.
Protrusions 29 are not in contact with each other, that is mutually isolated. The protrusions 29 are each extremely small, measuring 1–20 μm in diameter and 0.5–5.0 μ m in height. Therefore, during a subsequent substrate washing process, a heating process, or a film forming process, adherence between the protrusions 29 and underlying layer may deteriorate, thus causing the protrusions 29 to problematically flake off.
With this, therefore, a desired uneven structure cannot be formed in a reflecting electrode region, so that a desired optical property cannot be obtained for the reflecting electrode. That is, such the reflection electrode, if used in the reflection-type liquid crystal display, would give dark display or irregularities in brightness.
To prevent flake-off of the protrusions, also, it may be suggested that a coupling material be applied under the protrusions 29 to improve adherence. Under and below the protrusions 29, however, the TFT, the wiring lines, and a like are arranged, so that they may be adversely influenced by the coupling material, thus deteriorating reliability of the switching element. Therefore, the coupling material should not be used.